We analyzed the records on the reign style and the calendrical data presented in the Samguksagi (History of the Three Kingdoms) and the epigraph of the Three Kingdoms (Silla, Goguryeo, and Baekje) period in Korea (B.C. 57 – A.D. 935) to verify the chronological tables of the period that are currently in use. For the epigraph of the Three Kingdoms of Korea, we utilized the database provided by the National Research Institute of Cultural Heritage. By analyzing the records on the reign style, first, we found that the Yeonpyo (Chronological Table) of the Samguksagi is tabularized using the reign style of the Caowei for the period of Three Kingdoms of China (Caowei, Shuhan, and Sunwu) (A.D. 220 – 280). Second, we found that the reign style of the Silla was promulgated during the reign period of a king, which was usually promulgated when a king acceded in China. Third, we found that the reign styles presented in historical materials showed agreement with those of the Yeonpyo except for two cases in the Goguryeo. Concerning the records on the calendrical data, first, we confirmed that accession years of kings Michu, Beobheung, Seondeok, Minae, Gyeongmun, and Seongdeok. On the other hand, we found probable errors in the record of accession years for the kings Munja, Wideok, Aejang, and Heonan. Next, we found that the records of the length of a lunar month, leap month, and cyclic day showed agreement with the current chronological table except for several cases. In particular, cyclic days in the solar eclipse account had a relatively large number of discrepancies. We believe that these errors might have been caused by the inaccurate identification of a year in the records of the Three Kingdoms while determining the year by referring to Chinese historical documents. Finally, we found that the starting point was included while counting a year, at least from the Three Kingdoms period, similar to the current method of counting age in Korea. However, we point out that the starting point should be excluded when the term “after” is used in the expression. We believe that this study is very useful to verify the current chronological table of the Three Kingdoms period in Korea.

Korea has numerous astronomical resources, such as observational records, star maps, and a wealth of literature, covering the period from the Three Kingdoms (54 BC - 932 AD) to the Joseon Dynasty (1392 - 1910 AD). The research activities related to these resources have been limited to those by individual researchers. It is now necessary to conduct research by efficiently and systematically collecting and managing Korean astronomical records using an accessible Web environment. The purpose of this study is to complete a system that enables researchers systematically to collect and verify a large number of historical records related to astronomical phenomena in a Web environment. In 2017, a preliminary survey was conducted, and the requirements pertaining to an implementation target system were devised. In addition, a joint development plan was carried out by the developer, lasting three months in 2018. Although the system is relatively simple, it is the first system to be attempted in the historical astronomy field. In order to proceed with the systematic development, the software development methodology is applied to the entire process from deriving the requirements of researchers to completing the system. The completed system is verified through integrated function and performance tests. The functional test is repeated while modifying and testing the system based on various test scenarios. The performance test uses a performance measurement test tool that takes measurements by setting up a virtual operation environment. The developed system is now in normal operation after a one-year trial period. Researchers who become authorized to use the system can use it to verify the accuracy of data and to suggest improvements. The collected feedback will be reflected in future systems, and Korean astronomical records will be available for use internationally through a multilingual service.

We investigate the provenance and the changes in the timekeeping system focusing on official records such as almanacs and textbooks published by the government after the solar calendar was introduced. We found that the solar calendar and the 12-hour clock time first appeared in 1884 during Joseon dynasty, at that time the solar calendar was used at the open port in Busan to facilitate the exchanges with Japan. The 12-hour clock time first appeared in the『Hansung Sunbo』published by the government in 1884. We also found that the Joseon dynasty also used 12 diǎnzhōng or 12 diǎn. In addition, the term of the ‘Sigan’ first appeared in the first official academic textbook in August 1895, and the chapter related to time contained the information about 12-hour clock time instead of the 12 Shi. In 1908, the meaning of the solar time, the equation of time, and the differences in longitude with the adoption of Korean Standard Time were introduced. Meanwhile, the 24-hour clock time was first introduced in Joseon and applied to railway times in 1907. The 1946 almanac, the first issue after liberation, used the 12-hour clock time which uses ‘Sango’, ‘Hao’ and the 24-hour clock time started to be used from the following year and is still used to this day. Finally, the 12-hour clock time, which was introduced around 1884, was enacted as Article 44 of the law in 1900 and was revised again in 1905 and 1908. In Korea, the terms related to the time in the current astronomical calendar system were newly defined around 1884, 1896, and 1908, and gradually standardized through the establishment of laws.

We study the operation of a lunisolar calendar in Korea and its time data calculation method. The dates based on the lunisolar calendar have been conventionally used in Korea after the Gregorian calendar was introduced in 1896. With the Astronomy Act enacted in 2010, the lunisolar calendar is presently being used as an official calendar along with the Gregorian calendar. However, no institutionalized regulations have been provided on the time data calculation method by the lunisolar calendar. The Korea Astronomy and Space Science Institute very recently established the regulations on the lunisolar calendar operation in Korea. We introduce the regulations together with historical substances and analyze the time data calculated according to the regulations for 600 years from 1901 to 2500. From our study, we find that the value of ΔT (i.e., the difference between the terrestrial time and the universal time) is the most critical parameter causing uncertainty on the data. We also find that all new Moon days in the almanacs agree with our calculations since 1912. Meanwhile, we find that new Moon and winter solstice times are found to be very close to midnight in 38 and five cases, respectively. For instance, the new Moon time on January 14, 2097 is 0 h 0 min 8 s. In this case, deciding the first day (i.e., new moon day) in a lunar month is difficult because of the large uncertainty in the value of ΔT. Regarding with a lunar leap month, we find that the rules of inserting the leap month do not apply for 17 years. In conclusion, we believe that our findings are helpful in determining calendar days by using the lunisolar calendar.

The life and astronomical activity of Lee Deok-Seong (李德星, 1720-1794) was studied using various historical sources, including the astronomical almanac, Seungjeongwon-Ilgi (Daily records of Royal Secretariat of Joseon dynasty), and the Gwansang-Gam’s logbooks during Joseon dynasty (A.D. 1392– 1910). We present the results of the study including the following main findings. First, from the investigation of Lee’s family tree, we find that a number of his relatives were also astronomers, notably Samryeok-Gwan (三曆官, the post of calendrical calculation). Second, we find that he took part in the compilation of an annual astronomical almanac over a period of at least 16 years. His major achievements in the astronomy of the Joseon dynasty were to establish a new method of calendar-making calculation and to bring astronomical materials to the Joseon court through a visit to China. The Joseon dynasty enforced the Shixianli (時憲曆, a Chinese calendar made by Adam Shall) in 1654 without fully understanding the calendar. So an astronomer and an envoy were dispatched to China in order to master the intricacies of the calendar and to learn as much of Western science as was available in that time and place. Lee Deok-Seong worked at the Gwansang-Gam (觀象監, Royal Astronomical Bureau) during the reigns of King Yeongjo (英祖) and Jeongjo (正祖). As best as we can ascertain in relation with the calculations in the Shixian calendar, Lee visited China four times. During his trips and interactions, he learned a new method for calendar-making calculations, and introduced many Western-Chinese astronomical books to Joseon academia. Lee greatly improved the accuracy of calendrical calculations, even while simplifying the calculation process. With these achievements, he finally was promoted to the title of Sungrok-Daebu (崇祿 大夫), the third highest grade of royal official. In conclusion, history demonstrates that Lee Deok-Seong was one of the most outstanding astronomers in the late-Joseon dynasty.

We investigate the records related to the reign style and the calendar day from the epigraphs of the Goryeo dynasty (918 - 1392) in Korea in order to verify and supplement the sexagenary cycle data of the first day in the lunar month of the dynasty. The database of the National Research Institute of Culture Heritage contains a rubbed-copy image, transcription statement, and translation statement for Korean epigraphs as well as 775 epigraphs corresponding to the Goryeo dynasty. The epigraph records are valuable in that, during this time, they were written differently from other historical literature such as the Goryeosa (History of the Goryeo Dynasty), which was compiled in the next dynasty. We find that the Goryeo dynasty, in general, had adopted the reign styles of Chinese dynasties at that time. We also find 159 calendar day records all showing good agreement with the work of Ahn et al. except for dozens of records. Through this study, we can verify the reign styles and the calendar days of the Goryeo dynasty.

Sambok (三伏, Three Hottest Days) is the common designation of Chobok (初伏, Early Hot Day), Jungbok (中伏, Middle Hot Day), and Malbok (末伏, Late Hot Day), and widely known to be one of the Korean folk customs. Hence, Sambok is notated in Manseryeok (Ten Thousand-Year Almanac) and in the annual astronomical almanac published by Korea Astronomy and Space Science Institute. In this paper, we investigate the changes of Sambok in Korea based on various documents such as Joseonwangjosilok (朝鮮 王朝實錄, Annals of the Joseon Dynasty), Jeungbo-Jakryeoksik (增補作曆式, The Supplement of Manual for Calendar Making), astronomical almanacs, and so forth. According to Jeungbo-Jakryeoksik preserved in Kyujanggak Institute for Korean Studies, Chobok and Jungbok are defined as the third and fourth Gyeongil (庚日, The Day Starting with the Seventh Heavenly Stems in Sexagenary Cycles Assigned to Each Day) after the summer solstice, respectively, and Malbok is the first Gyeongil after Ipchu (Enthronement of Autumn). However, if the summer solstice is Gyeongil, then the third Gyeongil counting from the solstice becomes Chobok. Malbok depends on the time of Ipchu. Ipchu itself becomes Malbok if the time of Ipchu is in the morning, or next Gyeongil becomes Malbok if it is the afternoon. On the other hand, Malbok is defined as Ipchu itself regardless of its time according to Chiljeongbobeob (七政步法, Calculating Method for Sun, Moon, and Five Planets), Chubocheobryeo (推步捷例, Quick Examples for Calendrical Calculations), and so on. To verify the methods used to determine Sambok, we examined the record in the extant almanacs during the period of 1392 to 2100 for which the summer solstice or Ipchu is Gyeongil. As a result, we found a periodicity that if the time of Ipchu is in the morning, in general, the time is in the afternoon after two years and then is back into in the morning after nineteen years, i.e., the 2 + 19 years periodicity. However, we found the 2 + 17 years periodicity in some years. We also found that the Chobok method of Jeungbo-Jakryeoksik has been used since 1712, the thirty-eighth reign of King Sukjong (肅宗). In addition, we supposed that Malbok had been determined by the method like Chubocheobryeo since either 1846, the twelfth reign of King Heonjong (憲宗), or 1867, the fourth reign of King Gojong (高宗). At present, these methods of Sambok are customarily used without any legal basis. We, therefore, think that this study will help conventionalize the method defining Sambok in the future.

In this paper, we study the structure of the Daegyupyo (大圭表, Large Gnomon) of the early Joseon dynasty. A Gyupyo (圭表, Gnomon that is Guibiao as pronounced in Chinese) is composed of a Pyo (表, Biao as pronounced in Chinese) making a shadow and a Gyu (圭, Gui as pronounced in Chinese) measuring its length. It is known that the Daegyupyo with the 40-feet height was constructed between the sixteenth to seventeenth year of the King Sejong reign (1444 - 1445) on the basis of the record of Yuanshi (元史, the History of the Yuan Dynasty). By analyzing historical documents such as Joseonwangjosillok (朝鮮王朝實錄, the Annals of the Joseon Dynasty), Yuanshi, and Jegaryeoksangjip (諸家曆象集, a work written by Sunji Lee), we found a possibility that the Ji (池, a pond) on the Gyu was located in the north side of the Pyo. This structure is different from that in previous studies, but is in a good agreement with that of the 40-feet Guibiao remaining in Dengfeng (登封) of China. Regarding to the Hoengyang (橫梁, cross-bar), we suggest that it was set up by double 5-feet supporting arms apart from the north tip of the Pyo in the radial direction. The 3:4:5 ratio in a rectangular triangle was used to place the Heongyang on the top of the Pyo at a distance of 4-feet (3-feet) in the vertical (horizontal) direction. We also discuss the structural problem when the Hoengyang is positioned apart from the top of the Pyo by supporting arms. In conclusion, we think that this study should be useful in restoring the Daegyupyo of the Joseon dynasty.

In this paper, we have studied Sogyupyo (小圭表, small noon gnomon) of the Joseon dynasty. According to the Veritable Records of King Sejong (世宗, 1418 - 1450), Daegyupyo (大圭表, large noon gnomon) with a height of 40-feet [尺] was constructed by Jeong, Cho (鄭招) and his colleagues in 1435, and installed around Ganuidae (簡儀臺, platform of Ganui). On the contrary, the details of Sogyupyo are unknown although the shadow length measurements by Daegyupyo and Sogyupyo are found on the Veritable Records of King Myeongjong (明宗, 1545 - 1567). By analysing historical documents and performing experiments, we have investigated the construction details of Sogyupyo including its development year, manufacturer, and installation spot. We have found that Sogyupyo would be manufactured by King Sejong in 1440 and placed around Ganuidae. And Sogyupyo would be five times smaller than Daegyupyo, i.e., 8-feet. On the basis of experiments, we suggest that although it is smaller, Sogyupyo was equipped with a bar [橫梁] and a pin-hole projector [影符] like Daegyupyo in order to produce the observation precision presented in the Veritable Record of King Myeongjong.

Since the thirteenth century, large scale facilities and various instruments for astronomical observation were built and installed in East Asia. During the Yuan Dynasty, S. ti.ntai (Beijing astronomical observatory in the Yuan Dynasty, 司天臺) was built in Beijing in 1279. Various astronomical instruments, including Ganui (Jianyi, simplified armillary sphere, 簡儀), Yang-yi (upward hemisphere, 仰儀) and Gyupyo (gnomon, 圭表) were installed in this observatory. These astronomical instruments were modified and improved by researchers of the Joseon Dynasty. Ganuidae (Joseon astronomical observatory, 簡儀臺) was built in Gyeongbokgung (or Gyeongbok palace, 景福宮), Seoul. Its scale was 31 Cheok (Korean feet in the Joseon Dynasty, 尺) in height, 47 Cheok in length and 32 Cheok in width. Lee, Cheon (李蕆, 1376~1451), a responsible leader of Ganuidae project, set up various astronomical instruments with his colleagues. Ganui and Jeongbangan (direction-determining board, 正方案) were installed at the top of this observatory. Gyupyo was installed at the west side of this observatory and Honui (armillary sphere, 渾儀) and Honsang (celestial globe, 渾象) were installed in a small pavilion which was located next to Gyupyo. A decade after installation, this observatory was moved to the north-west side of the palace but almost destroyed during Japanese invasion of Korea in 1592 except Ganuidae. We have analyzed documents about Ganuidae and investigated Chinese remains of astronomical observatories and artifacts of astronomical instruments. In this paper, we suggest the appearance, structure, arrangement and scale of Ganuidae, which are expected to be used for the restoration of Ganuidae at some day in the near future.

Korea Astronomy and Space Science Institute (KASI) has constructed online database of the Korean Astronomical Almanac as a part of the 'Knowledge and Information Business Project 2009' supported by the Ministry of Knowledge Economy. Throughout this project, KASI provides both digitized images and extracted text from the almanac covering approximately 240 years. The primary purpose of this paper is to serve as a reference for users of the almanac database. Hence, we introduce the calendar history and kinds of the astronomical almanac used in Korea, and explain the contents of the almanacs according to its appearance time. We also briefly mention the steps employed in constructing the database system and Web site. Since ancient times, astronomical almanacs have been an essential part of daily life. We, therefore, believe that the astronomical almanac database constructed by KASI will prove its usefulness in various fields, and particularly in the study of historical astronomy.

In the beginning of the Joseon dynasty, the Royal Astronomical Bureau (觀象監, shortly RAB) was established. After the double RAB had settled down by King Sejong (世宗), it continued to function until 1907. Before the Japanese invasion of Korea in 1592, the Joseon court had the Inner RAB in the Gyeongbok Palace (景福宮) and the Outer RAB in the Northen District Gwangwha-Bang (北部廣化坊) at the western side of the Changdeuk Palace (昌德宮). In the reign of King Sukjong (肅宗) the double system of the RAB was transformed into the Geumho-Gate (金虎門) Outer RAB and the Gaeyang-Gate (開陽門) Outer RAB. During the reconstruction of the Gyeongbok Palace in the early reign of King Gojong (高宗), the Gaeyang-Gate Outer RAB was replaced by the Yeongchu-Gate (迎秋門) Outer RAB in 1865. All RAB had the Royal Astronomical Observatory (觀天臺, RAO hereinafter), so called the Soganui-platform (小簡儀臺) on which the Soganui (小簡儀) has been put. The Soganui (小簡儀) is a small simplified armillary sphere. While the Gwangwha-Bang RAO handed down from the reign of King Sejong still exists, other RAOs, such as Gyeongbok Palace RAO, Gaeyang-Gate and Yeongchu-Gate RAOs, do not remain. According to our study, the Changgyeong Palace (昌慶宮) RAO was not indeed the RAO with the Soganui.